Quality of Service (QoS) is both a measurement of system performance and an expectation that users or clients of a system have. Throughput, latency, error rates, availability and other parameters can be measured to determine system performance, and various systems and mechanisms have been devised to guarantee it. Many network, storage and computing systems implement Quality of Service with a queuing system. If an input/output (I/O) request is within a well-defined credit limit, then the I/O request is allowed to enter into the system, and otherwise the I/O request is queued until it has sufficient credits. These credits are replenished every second (or other time interval), so there is not any starvation. In virtualized storage systems, once entered into the system, the I/O request can go through the entire storage stack and then through the storage stack of an underlying hypervisor operating system and finally to a device queue. However, if the system allows the I/O request just on credits, or other such similar algorithm, it is very likely that many I/O requests can land at the same time at the device layer. Then, even after all of the efforts which a Quality of Service queuing system takes to control throughput and latency, the I/O requests can still congest a device. This can result in unpredictable latency. Also, this has a cascading effect because once latencies are high, the Quality of Service queuing system then drastically throttles back the I/O requests allowed through, and on the next clock tick the system observes low latency and increases the capacity of the system again. This can result in an oscillation of latency and throughput, which is undesirable and fails to meet latency guarantees consistently in a Quality of Service system. There are also difficulties in supporting different deadline classes with different urgencies of I/Os. Therefore, there is a need in the art for a solution which overcomes the drawbacks described above and satisfies both latency as well as throughput sensitive applications regardless of priority.